The invention relates to a combined heat exchanger, particularly for a motor vehicle, including a bank of tubes linked to manifolds and divided into two parts capable of being traversed by different fluids.
In a heat exchanger of this type, the two fluids are cooled by the same airflow which sweeps through the bank.
The invention relates more particularly to a combined heat exchanger in which the bank of tubes is divided into a part forming an oil cooler, the tubes of which are suitable for being traversed by oil, and into a part forming a condenser, the tubes of which are suitable for being traversed by a cooling fluid.
In such a heat exchanger, the oil is typically the transmission oil, in particular for an automatic gearbox of a motor vehicle. As for the condenser, it serves to cool the cooling fluid for a motor vehicle air-conditioning installation.
At the present time, the cooling of the cooling fluid and the cooling of the transmission oil are carried out by two separate exchangers, usually a parallel-flow condenser and an oil exchanger, of the vane type, placed in proximity to the condenser.
It is known, moreover, according to the Japanese Utility Model No. 61-167202 to produce a combined heat exchanger comprising a part forming a condenser and a part forming a heat exchanger. This heat exchanger comprises a common bank of tubes linked to two tubular manifolds.
The production of a combined heat exchanger, including a part forming an oil cooler and a part forming a condenser, poses many problems because the two fluids exhibit very different characteristics. Thus, the viscosity of the oil is very much greater than that of the coolant and the loss of pressure head in the oil is therefore very high.
Moreover, the two fluids circulate at very different temperatures, that of the oil being very much higher than that of the cooling fluid. These substantial temperature differences are capable of engendering differential-expansion phenomena which may damage the heat exchanger and lead to leakage.
Moreover, it may happen that the cooling fluid is heated by the oil, which then leads to a degradation in performance of the condenser part.
The invention aims to afford a solution to the above problems.
According to the present invention there is provided a combined heat exchanger including a bank of tubes linked to manifolds and divided into a part forming an oil cooler, the tubes of which are suitable for being traversed by oil, and into a part forming a condenser, the tubes of which are suitable for being traversed by a cooling fluid, wherein the tubes of the oil-cooler part and the tubes of the condenser part are different and possess respective hydraulic diameters related by the following inequality:
0.8 mm2xe2x89xa6DHaxc3x97DHbxe2x89xa63.00 mm2
where the hydraulic diameter DH of a tube is defined by the formula DH=4S/P, in which S designates the area of the cross-section of the tube (expressed in mm2) and P the internal perimeter, or xe2x80x9cwet perimeterxe2x80x9d, of the tube (expressed in mm).
Hence, the combined heat exchanger of the invention comprises different tubes, that is to say that the tubes of the condenser part are adapted to the circulation of the cooling fluid, whereas the tubes of the oil-cooler part are adapted to the circulation of the oil.
Moreover, it is essential for the product of the respective hydraulic diameters DHa and DHb to satisfy the foregoing inequality relationship. This is because it has been observed that when the product DHaxc3x97DHb is greater than 3.00 mm, the thermal power exchange within each of the two fluids drops off significantly. Moreover, when this product is less than 0.8 mm2, the loss of pressure head in the oil circuit increases vary greatly.
In the invention, the tubes of the bank are advantageously multi-channel tubes.
The hydraulic diameter of the tubes of the oil-cooler part is preferably greater than the hydraulic diameter of the tubes of the condenser part.
It is particularly advantageous for the number of channels of the tubes of the oil-cooler part to be less than the number of channels of the tubes of the condenser part. This means, in other words, that the tubes of the oil-cooler part contain fewer partitions than the tubes of the condenser part. This makes it possible to increase the hydraulic diameter and thus significantly to lower the loss of pressure head generated by the circulation of the oil in these tubes.
The tubes of the bank are advantageously obtained by extrusion.
According to another characteristic of the invention, the tubes of the bank are linked to two manifolds each of which includes a separating partition for isolating the oil circulating in the oil-cooler part and the cooling fluid circulating in the condenser part.
Taking into account the differences in temperatures between these two fluids, there is a benefit in using partitions forming thermal insulation.
According to yet another characteristic of the invention, the heat exchanger comprises means forming a thermal barrier between the tubes of the oil-cooler part and the tubes of the condenser part.
These means make it possible to limit the stresses due to the phenomena of differential expansion and to prevent the cooling fluid being heated by the oil, which is at a very much higher temperature.
In one embodiment of the invention, the means forming a thermal barrier comprise a tube of the bank, called xe2x80x9cinactive tubexe2x80x9d or xe2x80x9cdead tubexe2x80x9d, which is not traversed by any fluid and which opens out between double partitions of each of the manifolds.
In another embodiment of the invention, in which corrugated spacers are provided between the tubes of the bank, the thermal-barrier forming means comprise a region devoid of corrugated spacers, which extends between two adjacent tubes belonging respectively to the oil-cooler part and to the condenser part.
According to another characteristic of the invention, the bank and the manifolds are assembled by brazing.
Hence, the combined heat exchanger of the invention can be produced according to the well-known technology of brazed exchangers, such as that used, for example, in the production of the condensers.